The invention relates to a control device for controlling the flow of oil to and from two control chambers of a piston having variable compression height and comprising an outer piston part and an inner piston part guided axially slidably relative to the outer piston part and articulated to a connecting-rod, the two control chambers being formed axially opposite one another and separated by the inner piston part, the first control chamber being arranged between a first surface of the inner piston part and a surface of the outer piston which faces a piston crown, and the second control chamber being arranged between a second surface of the inner piston part and a surface which faces a shank of the outer piston part.
Pistons having variable compression height are known and described in "MTZ Motortechnische Zeitschrift 47 (1986) 5" for example. Pistons of this type exhibit an inner piston part connected positively to the crankshaft and an outer piston part connected frictionally to it through a hydraulic system, and arranged slidably on the inner piston part. Two control chambers connected by oil bores are arranged between the inner piston part and the outer piston part.
During the exhaust stroke the outer piston part is drawn by inertia, so that a pressure increase is caused in the lower control chamber. This causes oil to flow through a throttle from the lower into the upper control chamber. In addition, the increase in volume of the upper control chamber causes a flow of oil from the gudgeon pin through a nonreturn valve into said control chamber. The result is an increase in the compression height of the piston.
After the mixture has ignited in the combustion chamber, resulting in an increase in gas pressure, this gas pressure is transmitted to the upper control chamber and delivers oil under control through a pressure limiter valve into the crankcase. Oil flows simultaneously from the upper control chamber through a throttle and through a nonreturn valve into the lower control chamber. The volume of the upper control chamber, and hence the compression height, are reduced in this manner.
However, a reduction of the compression height during combustion is undesirable during the cold-start and the warm-up phase, and is disadvantageous due to less favorable ignition conditions and a longer warm-up phase with increased toxic emission.
Starting from the prior art, it is an object of the invention to construct a device of the generic type so that the operating behavior of the internal-combustion engine is improved by maintaining a great compression height during the cold-start and in the warm-up phase.
This object is achieved according to the invention by providing an arrangement wherein said oil flow control device comprises:
a first oil bore in the inner piston part which connects the first control chamber to an oil supply means and contains a nonreturn valve which opens towards the first control chamber,
a second oil bore in the inner piston part which connects the first control chamber to a crankcase and which contains a pressure limiter valve which opens towards the crankcase,
a third oil bore in the inner piston part with the purpose of controlling the flow of oil between the two control chambers,
and a control element which closes the second oil bore when the engine is cold and clears it in the warm operating state of the engine.
In especially preferred embodiments the control element is an expansible element which exhibits minimal length in the cold start and has a control slide valve connected to it.
In a piston of the above-described type a control element is inserted in the run of the oil bore between the upper control chamber and the crankcase. This control element consists of a control slide valve which closes the oil bore when the engine is cold to thereby prevent the discharge of oil from the upper control chamber into the crankcase and thus provide a constant compression height. As the operating temperature of the engine rises the control slide valve gradually clears the flow of oil again through the oil bore after ignition of the mixture and resulting pressure rise in the upper control chamber, so that a reduction of the compression height can occur. The control slide valve is connected mechanically to an expansible element fastened in the inner piston part, so that when the engine is cold the expansible element maintains the control slide valve in such a position that the oil bore is completely closed by the control slide valve. With rising engine operating temperature, the expansible element, due to its change in dimension, moves the control slide valve out of the oil bore so that a control of the discharge of oil through the oil bore into the crankcase occurs. In the warm operating state of the engine the control slide valve has assumed a position in which the oil bore cross-section is fully cleared.
With the control device of the present invention, improved starting behavior is obtained when the engine is cold, and reduced toxic emission is achieved by shortening the warm-up phase.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.